Radiation of different wavelengths, such as visible and infrared radiation, is focused at different focal points by a given lens. For example, radiation of relatively short wavelength, such as visible light, will focus at a point nearer to a lens than radiation of relatively long wavelength, such as infrared radiation. Further, the size of the image focused at the focal point is a function of the distance from the focal point to the lens. Thus, in an optical system where the object is illuminated by radiation of different wavelengths, the visible image and the infrared image, if both are in focus, will be noncoincident in space and of different magnifications.
Some optical inspection systems may project images by using radiation of different wavelengths and may transmit the images along a common path through common lenses. In such a case, a limit is imposed upon the degree of precision or resolution of the system by the noncoincidence of the focused images and by the different magnifications of the focused images resulting from the different wavelengths used.